This invention relates to a microwave circuit apparatus. More particularly, the present invention relates to a microwave circuit apparatus suitable for a self-bias circuit of a functional element.
As is known generally, bias circuits are classified as a self-bias circuit or a fixed bias circuit, and these two types of circuits are suitably employed depending upon the intended application.
If a microwave circuit apparatus such as a microwave amplifier, for example, is constructed by the conventional self-bias circuit, the grounding effect of the electrode of a functional element to be grounded for high frequencies drops and the microwave apparatus fails to provide sufficient electric performance. On the other hand, if the microwave amplifier is constructed by the fixed bias circuit, the performance can be improved but positive and negative power sources must be used so that the apparatus becomes more complicated in construction and more expensive to produce. Accordingly, a microwave circuit apparatus that can provide sufficient electric performance even when applied to the self-bias circuit having a simple construction has long been desired.
In forming the microwave circuit apparatuses in general such as the microwave amplifier, a GaAs field effect transistor (hereinafter referred to as the "FET") having excellent high frequency characteristics has been disposed on a dielectric substrate such as a ceramic substrate together with other circuit elements.
Typical examples of the conventional microwave circuit apparatus of this kind are illustrated in FIGS. 1 through 4. FIG. 1 shows a conventional microwave circuit apparatus in which self-bias is applied to the FET. Reference numerals 1, 2, 3 and 4 represent microstrip lines for impedance matching or for transmission, and reference numerals 6 and 7 represent an impedance matching capacitor or a d.c. blocking capacitor. Reference numeral 8 represents a bypass capacitor for connecting the source S of the FET 5 to the microstrip line 12 by high frequencies on the grounding side and reference numeral 9 represents a bias resistor which generates a bias voltage necessary for the operation of the FET 5. Reference numeral 10 represents a gate resistor for applying the bias voltage to the gate G of the FET 5 and reference numeral 11 represents a drain resistor for applying the power source voltage B.sub.1 to the drain D of the FET 5. Reference numeral 12 represents a microstrip line which functions as a high frequency ground and also as the ground with respect to the power source B.sub.1. All these circuit elements are formed on a dielectric substrate by printing or like means as shown in FIGS. 2A and 2B. Reference numeral 5 represents the FET as the functional element and symbols IN and OUT represent the input and output terminals of the microwave circuit apparatus, respectively.
In the conventional microwave circuit apparatus shown in FIG. 1, high frequencies at the source S of the FET 5 are grounded to the microstrip line 12 by the capacitor 8 and its route extends from the electrode 8B forming the capacitor 8.fwdarw.electrode 8A.fwdarw.dielectric 8C.fwdarw.electrode 8D.fwdarw.through-hole 26.fwdarw.microstrip line 12. Hence, the distance from the source S to the microstrip line 12 is large and the high frequency loss by the electrodes 8B, 8A and 8D increases. Furthermore, since the capacitor 8 is disposed on the surface of the dielectric substrate 27 together with the FET 5 and the other circuit elements, its occupying space is limited and the dielectric loss of the dielectric 8C also increases. In other words, an inductance 8' and a resistor 8" exist in the grounding route of the source S of the FET 5, as illustrated in the equivalent circuit diagram of FIG. 3. As a result, a part of the output of the FET is fed back to the input through the source S so that when the microwave circuit apparatus is a microwave amplifier, sufficient gain can not be obtained. Moreover, operational stability as the amplifier drops and high frequency parasitic oscillation is likely to occur.
FIG. 4 shows a fixed bias circuit which has gained a wide application in microwave circuit apparatuses as a circuit that eliminates the problems described above. Like reference numerals are used in FIG. 4 to identify like constituents as in FIG. 1. The circuit portion 14 encircled by dotted line in FIG. 4 is one that obtains a negative bias voltage B.sub.2 necessary for the gate G of the FET 5 from a single positive power source B.sub.1. The circuit portion 14 consists of an astable multivibrator 16 and a rectification-smoothing circuit 17. The circuit portion 13 encircled by another dotted line is one that stabilizes the bias of the FET 5. A voltage necessary for the gate G and drain D of the FET is fed thereto through this circuit portion 13. The source S of the FET 5 is directly connected to the microstrip line 12 on the grounding side, thus forming the microwave circuit apparatus. The circuit shown in FIG. 4 is also disposed on the dielectric substrate in the same way as the circuit shown in FIG. 1.
Microwave circuit apparatuses for satellite communication are used outdoors as the outdoor units and in order to simplify the feed of power, only a single, positive power source is generally applied to the outdoor unit while a negative power source is generated inside the unit. This system is shown in FIG. 4.
The embodiment shown in FIG. 4 has an excellent high frequency grounding effect of the source S but the circuit portions 13 and 14 make the apparatus as a whole further complicated. Accordingly, the microwave circuit apparatus can not be made compact, the number of components increases and various problems occur such as drop of reliability, lowering of the assembly working factor, increase in the cost of components and in the power consumption that in turn results in the increase in the maintenance cost, and so forth.